This application is a U.S. National Phase Application of PCT/NO00/00449 filed on Dec. 22, 2000, claiming priority to Norwegian Application No. 19996530 filed on Dec. 28, 1999.
This invention relates to a torque release coupling for use in drill strings comprising an outer string part rotatably mounted outside a radially inner string part, and a momentum limited rotation lock positioned between them.
In drilling operations, especially related to oil and gas production, long drill strings are used penetrating different types of geological formations with varying hardness and drilling resistance. The drill strings may consist of a number of sections with decreasing diameter downward in the drilled hole, and thus it is difficult to decide how large of a momentum the drill string may be subjected to during drilling. If the momentum is too large the drill string may be subject to damage, e.g. in the joints in the drill string, which results in that it must be removed from the drilled hole to be repaired. This is related to delays and large costs.
These problems may be solved using torque release couplings positioned along the drill string hindering that torque over a certain limit is transmitted along the string. A number of devices for obtaining such couplings are known, comprising two parts rotating relative to each other with a rotation limiter between them, but none of these have been usable in practice. Thus there are no such devices available at this time.
The rotation limiting devices may be breakable bolts being adapted to break when the torque exceeds a certain limit. This solution has the disadvantage that the bolt must be replaced after use, so that the drilling still has to be interrupted. The damage to the coupling is, however, limited.
Another type of a rotation limiting device is friction surfaces being held together with a force. This solution does, however, have the disadvantage that the friction is difficult to predict in practice, and will also change for each time the coupling has been in action.
In EP 151,365 a solution is described in which a split ring is positioned between an upper and a lower part of the drill string and is fastened in the first part with a locking pin, the split ring being adapted to rotate relative to the second part if the torque exceeds a certain limit. This represents a rather complicated solution which also is based on the difficulty in predicting the friction between the parts of the coupling.
U.S. Pat. No. 5,137,087 describes a cementing tool with a torque release coupling comprising a sheath with a number of teeth with inclined side surfaces being held together by a spring. When the coupling is subject to a sufficiently large torque the axial force exceeds the spring force and the two parts may rotate relative to each other. This is, however, a tool being meant for temporary positioning in the well and must be pulled out of the well afterwards. Thus it is not suitable for use as a part of the drill string, where stricter requirements must be held to the repeatability of the coupling and the coupling must allow fluids from the well to flow through it.
It is thus an object of this invention to provide a torque release coupling as described above in which the release torque is predictable and reproducible, based on a relatively simple design. The invention is characterized as stated in the independent claim.